Three sites on a crawling neuron produce the force that pushes it along, Jiang et al. reveal.

Traction force propels a cell forward or backward. One study of fibroblasts found that a single location called the contraction center was responsible for the cell’s traction force, but where a migrating neuron, with its small cell body and long projections, generates traction remains unclear.

The researchers used traction force microscopy to map the forces generated by cerebellar granule cells. They found that a granule cell has three contraction centers: just behind the tip of its forward projection, at the base of this projection, and in its tail. Myosin II activity and F-actin polymerization produced the force at each contraction center. However, the researchers found that microtubules dampened force generation, perhaps because they stiffen the cell.

All three contraction centers can operate at the same time, so Jiang et al. investigated how cells coordinate their activity. Granule cells crawl toward brain-derived neurotrophic factor (BDNF) and recoil from the protein Slit2. When Jiang et al. placed BDNF at the front of a neuron’s cell body, the location of the strongest contraction center shifted forward and the neuron moved ahead. But when they placed Slit1 at the front of the cell body, the location of the strongest center shifted toward the rear and the cell went into reverse. The researchers conclude that, depending on its course, the cell adjusts the forces produced by each contraction center.